1. Field of the Invention
This invention generally relates to a composition, process, and apparatus for purifying inert gas mixtures to remove Lewis acid and oxidant impurities therefrom.
2. Description of the Related Art
The provision of high purity inert gas streams is critically important in a wide variety of industrial and research applications.
An example of such applications is the fabrication of semiconductor devices. A number of inert gases are employed in such fabrication, and these gases as supplied frequently are contaminated with impurities such as Lewis acids and/or oxidants, e.g., oxygen and/or water. Such impurities when present in any significant amounts can render the product semiconductor device deficient or even useless for its intended purpose.
Accordingly, there is a continuing need in the art to develop and deploy improved apparatus, processes, and materials for purifying inert gas streams of such impurities.
The formation of scavengers suitable for purifying aprotic fluids such as aliphatic hydrocarbons, olefins, and gases used in the semiconductor industry, by pyrolyzing Group I metal alkyls on an inorganic support to convert the metal alkyls to their corresponding metal hydrides, is disclosed in "Metal Hydride Scavengers for the Removal of Impurities from Aprotic Fluids," Research Disclosure No. 276105, of Hercules Incorporated, April, 1987. A criterion for the use of such scavengers is that the fluid to be purified be inert to the scavenger. The capacity disclosed for this scavenger, in terms of oxygen impurity removal capacity, is two liters of oxygen per liter of a bed of such scavenger.
U.S. Pat. Nos. 4,604,270 and 4,603,148 to G. M. Tom disclose scavengers in which alkyl metal compounds are immobilized by coupling them to an organic polymeric support, followed by pyrolysis to yield a dispersed phase of the metal hydride in the organic polymer matrix.
Our U.S. Pat. No. 4,761,395 issued Aug. 2, 1988 and entitled "Process and Composition for Purifying Arsine, Phosphine, Ammonia, and Inert Gases to Remove Lewis Acid and Oxidant Impurities Therefrom," discloses scavenger compositions comprising a support having associated therewith, but not covalently bound thereto, an anion which is reactive with impurities in the gas mixture to remove same. The anion may be a carbanion whose corresponding protonated compound has a pK.sub.a value of from about 22 to about 36, and/or anions formed by reaction of such carbanions with the primary component of the gas mixture, i.e., carbanions that selectively deprotonate the primary component of the gas mixture, such as arsine, phosphine, or ammonia, to form the corresponding arside, phosphide, or amide anion as the active scavenging species. An illustrative scavenging source compound loaded on the support to form such prior art scavengers is triphenylmethyllithium, which provides a triphenylmethide anion as an active scavenging moiety.
H. L. Rhodes, "An Apparatus and Procedure for Calibrating a Water Vapor Analyzer in the 0.1 to 15 ppm Range," Report of Investigations No. 8548, U.S. Department of the Interior, Bureau of Mines (1981), describes a method of detecting small amounts of water in a gas, wherein bulk powdered calcium hydride is reacted with the water vapor at 150.degree. C. to form hydrogen gas, which then is analyzed by gas chromomatography to determine the original water content of the gas containing same.
The absorption of water and carbon dioxide on bulk lithium hydride is reported in Smyrl, M. R., et al, "Monitoring the Heterogeneous Reaction of LiH and LiOH with H.sub.2 O and CO.sub.2 by Diffuse Reflectance Infrared Fourier Transform Spectroscopy," Applied Spectroscopy, 37, pp. 38-44 (1983).
U.S. Pat. No. 2,919,268 to P. Fotis, Jr., et al discloses a composition formed by heating a Group IIa metal and a metal oxide at a 0.15-1.0 weight ratio and a temperature of 500.degree.-700.degree. C., to form a catalyst said to be useful in the polymerization of normal alkenes.
U.S. Pat. No. 4,450,099 discloses a synthesis gas catalyst formed by slurrying barium hydroxide with a metal oxide such as barium oxide, alumina, and silica, followed by calcining of the resulting solids at 200.degree.-900.degree. C.
It is an object of the present invention to provide an improved scavenger for purification of inert gases to remove Lewis acid and oxidant impurities therefrom.
It is another object of the present invention to provide an improved scavenger of such type, which is characterized by high scavenging capacity, e.g., in terms of water and oxygen removal capacity.
It is still another object of the invention to provide a scavenger of the aforementioned type, which is readily formed at comparatively low temperatures. It is a further object of the present invention to provide a method of making the aforementioned catalyst, and a process and apparatus for utilizing same to purify inert gas mixtures, to remove Lewis acid and oxidant impurities therefrom. Other objects and advantages will be more fully apparent from the ensuing disclosure and appended claims.